This invention relates to a shock absorber for a motorcycle such as a two-wheeled or three-wheeled motorcycle or the like.
A conventional shock absorber for a rear wheel of a motorcycle known hitherto is, as disclosed in the unexamined Japanese utility model registration application JIKKAISHO 61-113084, generally such that it comprises a coiled spring and a linear expansion type damper concentrially combined with the spring, and a swing arm and a frame assembly are connected to each other thereby in the vertical direction. There has also been known another type such as disclosed in the unexamined Japanese patent application TOKKAISHO 56-28008 in which there is interposed a coiled spring between a swing arm and a frame assembly as in the foregoing and a rotary type hydraulic damper is employed as a damper therein, said rotary hydraulic damper being so installed as to be concentric with the pivot of the swing arm or positioned right above said pivot. There has also been known still another type such as proposed by the applicant of this application in the unexamined Japanese patent application TOKKAISHO 62-46037 in which a rotary type hydraulic damper is connected to a frame assembly and to a swing arm via a link mechanism, and the swing arm is pivotally supported on the frame assembly through a torsion bar.
Of these prior art examples, the first linear expansion type shock absorber has to use a long coiled spring together with a long-sized hydraulic damper to make its stroke long if it is to maintain good shock-absorbing performance. With their lengths increased as above, there result such problems as an increased weight and an increased height of the gravitational center.
In the conventional shock absorber disclosed in the second prior art example above, the coiled spring is interposed between the swing arm and the frame assembly as in the first prior art example above, so that said shock absorber should have the same problems as the former in terms of the weight and the center of gravity of the coiled spring. In the third example of the prior art, the torsion bar should provide dual functions, one as a pivot pin and the other as a spring, so that it is difficult to design such a torsion bar serving the two purposes.
In addition, in the first and second prior art examples, the shock absorber occupies the under-frame space up to the upper end thereof, thus leaving a lesser space for installing an air cleaner and an oil tank or the like of the engine, which is another problem or disadvantage.
Therefore, it is much desired to have a shock absorber for a motorcycle which can be mounted on so low a position as to help significantly lower the height of gravitational center of a motorcycle and which is so small-sized as to leave thereabove enough space for installing other components.
Furthermore, the rotary type hydraulic damper mounted concentrically with the pivot of the swing arm in this type of the absorber can only swing for the same magnitude of angle as the swing arm can, and that mounted right above the pivot also has about the same swing angle as that of the swing arm because it is connected to the swing arm to have about the same arm length. Thus in either case, the stroke is small. Because of this, it is difficult to cause the oil to flow in a sufficient amount necessary to effect damping of shocks or vibrations. This makes it necessary to either reduce the cylinder volume or provide double oil chambers. It is thus unavoidable to make the shock absorber rather large-sized and more complex.
It is, therefore, strongly desired to obtain a shock absorber for a motorcycle which is small-sized to be mounted at a lower position and which is efficient enough to provide effective buffer action.
In the foregoing hydraulic damper, whether it is an linear expansion type or a rotary type, the vibration damping action is constant regardless of the damper being located above or below the frame if speed of vibration is equal. Therefore, in the bottoming position where a large load is applied, that is, in the neighborhood of the lower limit position of the frame assembly, the buffer action becomes insufficient so that the riding quality or comfort becomes poor.
As a means to improve this riding quality or comfort, it has been known to increase the cushion ratio which is ratio of a load applying to the swing arm at the upper-limit position of the frame assembly where the coiled spring is fully expanded to a load acting on the damper at the lower-limit position of the frame assembly. However, there has not been known as yet a simple mechanism therefor which brings about the above desired effect.